According to the Association for Science in Autism Treatment,
"Autism is a complex neurobehavioral disorder characterized by impairment in reciprocal social interaction, impairment in communication, and the presence of repetitive and stereotypic patterns of behaviors, interests, and activities. The onset of symptoms is typically before the age of 3 years. The severity of impairment in the given domains as well as the pattern of impairments varies from individual to individual; that is why diagnosticians refer to a “ spectrum ” of disability.

Impairment in social interaction range from difficulty initiating and maintaining interaction, impaired ability to recognize and experience emotions, and difficulty processing and appreciating the thoughts and feelings of others. Communication deficits range from no useful form of communication to very advanced language abilities, but little ability to use language in a social manner. Repetitive and stereotypic behaviors include perseverative behaviors such as complex rituals, extreme difficulty adapting to change and transition, and unusual movements such as hand flapping or whirling."

The National Institute of Mental Health lists the following as Possible Indicators of Autism Spectrum Disorders:
* Does not babble, point, or make meaningful gestures by 1 year of age
* Does not speak one word by 16 months
* Does not combine two words by 2 years
* Does not respond to name
* Loses language or social skills

Other Indicators

* Poor eye contact
* Doesn't seem to know how to play with toys
* Excessively lines up toys or other objects
* Is attached to one particular toy or object
* Doesn't smile
* At times seems to be hearing impaired